just ignore them the notices are not enforceable unless they go to the County court and then they will only be awarded the actual loss that they have suffered. As it costs them around £1500 to put the case to court and they will be very lucky to be awarded £1 nominal damages they are not going to pursue it.
You will get nasty letters but that is as far as it will go, just bin the letters without opening them. They give up after around 6 weeks.